According to the Rayleigh criterion, the lateral resolution which can be obtained with classical optical microscopy techniques is limited to about half the optical wavelength, which corresponds to about 250 nm in the visible optical wavelength range.
In order to overcome this limitation, several techniques have been developed, such as the atomic force microscopy (AFM), the scanning tunneling microscopy (STM), and the near-field optical microscopy, usually called SNOM for “Scanning Near-field Optical Microscopy”.
Near-field optical microscopy allows reaching resolutions in the order of hundreds of nanometers or even tens of nanometers, and studying objects with dimensions smaller than the wavelength. It also makes it possible to obtain simultaneously optical density images and 3D topology images of the surface of the object. It finds for instance applications in physics and biology research.
The key element in near-field optical microscopy is an optical probe which comprises a tip and, at the end of the tip, an optical aperture with dimensions in the order of tens of nanometer. The probe is positioned with the optical aperture very close to the surface of the object under measurement, at a distance down to a few nanometers or even in contact. At such distance a coupling by evanescent optical fields appears between the object and the probe, which is the basis of the measurement.
The spatial resolution is determined by the size of the optical aperture of the probe rather than by the wavelength of the light as in classical optical microscopy: the SNOM technique allows imaging features with size comparable to the size of the aperture of the probe.
An image representative of the coupling can be obtained by scanning the surface of the object with the probe. During the scanning, the height of the probe relative to the surface of the object (or at least a control parameter) is usually maintained as much as possible constant by means of a closed loop control system driving for instance a piezo actuator which moves the probe. The 3D topology image of the object surface can be obtained from these probe displacements.
In addition to imaging, the technique allows interacting with the surface of an object or even manipulating it.
Measurements can be done essentially in two modes: an illumination mode or a collection mode.
In illumination mode, light is emitted by the probe whose optical aperture behaves as a local source of evanescent waves. These waves are diffracted by the patterns of the object whose size is comparable to the size of the optical aperture of the probe, producing propagating waves which propagate through the object and can be detected.
In collection mode, the object is illuminated by propagating optical waves outside the probe, usually through the object. These waves are diffracted by the patterns at the surface of the object, causing evanescent fields to appear in the vicinity of these patterns. The evanescent waves produced by patterns with a size comparable to the size of the optical aperture of the probe can couple into it, producing propagating waves into the probe tip which can be detected.
The probes are currently basically of the fiber type or of the cantilever type.
The probes of the fiber type consist essentially in an optical fiber. The end of the fiber, constituting the probe itself, is sharpened (for instance using a wet etching process) to form a conical tip with a radius of curvature at its extremity down to a few tens of nanometers. The extremity of the tip is the optical aperture. The tip, except the optical aperture, is usually metalized. These probes represent by far (about 80%) the majority of the probes used in commercially available SNOMs. They can be used in collection mode and in illumination mode, and have transmission coefficients in the order of 10−4. They are usually based on silica glass fibers, which are a quite cheap material. The fabrication technique is simple but rather adapted to low-volume or unit production. Its reproducibility is quite low, with as a result a quite large dispersion in the product specifications. In addition, these probes are very fragile.
The probes of the cantilever type comprise a hollow tip or a tip in light-transmitting material held by a cantilever. The extremity of the tip forms an optical aperture with a diameter in the order of 100 nm. These probes are similar to those used in Atomic Force Microscopy (AFM). The ones which are currently commercially available can only be used in illumination mode. The transmission coefficient of the nano aperture is in the order of 10−4. These probes are usually fabricated by batch process, using photolithography and others silicon wafer processing techniques in use in microelectronics. So the optical and mechanical specifications of the cantilever probes show much less dispersion than the specifications of the fiber probes. And, because of their geometry, these are also less fragile.
In contrast with the fiber type probes, the probes of the cantilever type offer interesting possibilities for the integration of passive or active optical functions on the cantilever, so as to turn them to MOEMS (Micro Opto Electro Mechanical System). Several publications have been made on that topic, proposing the use of various materials, various fabrication techniques and various kinds of functions and components. For instance, materials such as silicon, silicon oxide, silicon nitride, InP, and integration of components such as waveguides, Schottky detectors, VCSEL laser diodes have been proposed.
We know for instance the paper from P. Gall-Borrut, B. Belier, P. Falgayrettes, M. Castagne, C. Bergaud, P. Temple-Boyer, “Silicon technology-based micro-systems for atomic force microscopy/photon tunnelling microscopy”, Journal of Microscopy, Vol. 202, Pt 1, April 2001, pp. 34-38, which discloses a probe of the cantilever type featuring a cantilever in silicon nitride (SiNx) which acts as a waveguide. The cantilever is bounded to a photodetector on a holder side opposite to the tip, with the waveguide guiding light between the tip and the photodetector.
Up to now however, the integrated optical functions remain very basic and do not extend far beyond interconnection of sources and/or detectors with the tip. In particular, no elaborated passive functions such as wavelength de-multiplexing or filtering have been done efficiently.
Reasons for that may be found in the materials used and the fabrication techniques, which are interdependent:
The probe materials must be compatible with the design of planar optics waveguide of complex shapes for instance, which is not the case of all the materials used;
The fabrication techniques used with materials such as silicon, when based on photolithography or similar techniques, involve very high production costs and, to be acceptable, high-volume production. So they are not compatible with low-volume or on-demand production of probes featuring specific functions.
Attempts have been made to design optical probes of the cantilever type using polymer materials and much less expensive fabrication techniques.
We know for instance the paper from H. Stürmer, J. M. Köhler, T. M. Jovin, “Microstructured polymer tips for scanning near-field optical microscopy”, Ultramicroscopy, vol. 71, 1998, pp. 107-110 which discloses a AFM/SNOM cantilever made in PMMA (Polymethylmethacrylate). The tip is also made in polymer and includes a fluorescent dye. However, the device do not feature any light guiding structure.
In another hand, we know techniques using sol-gel organo-mineral materials which are used for the realization of integrated optical circuits. The organo-mineral material is deposited on a substrate such as a silicon wafer, and optical structures such as waveguides are realized through a local polymerization process using UV light exposure, which also modifies locally the index of refraction. Components are then connected to optical fibers for interfacing. The technique is used for instance for manufacturing telecom optical components or chemical sensors.
It is an object of the invention to provide a method for manufacturing AFM/SNOM probes allowing the integration of elaborated passive and/or active optical functions on the probe.
It is also an object of the invention to provide a manufacturing process of such probes allowing versatility and flexibility in the design of the probes and low production costs at low volumes.